1. Technical Field
The present invention relates to an electron-beam device, in particular a scanning electron microscope, having a beam generator for generating an electron beam, an objective lens for focusing the electron beam on an object, and at least one detector for detecting electrons scattered on the object or emitted by the object. Furthermore, the present invention relates to a detector system for detecting electrons, in particular for an electron-beam device (for example, a scanning electron microscope).
2. Description of the Related Art
Electron-beam devices, in particular scanning electron microscopes, are used to examine surfaces of objects (specimens). For this purpose, an electron beam (hereinafter called primary electron beam) is generated in a scanning electron microscope using the beam generator and is focused through the objective lens onto the object to be examined. Using a deflection device, the primary electron beam is guided to scan the surface of the object to be examined. In the process, the electrons of the primary electron beam interact with the object. As a consequence of the interaction, electrons are in particular emitted from the surface of the object (secondary electrons) or electrons of the primary electron beam are backscattered (backscattered electrons). Here, the backscattered electrons exhibit energy in the region of 50 eV up to the energy of the electrons of the primary electron beam at the object, while the secondary electrons exhibit energy of less than 50 eV. Secondary and backscattered electrons form the secondary beam and are detected via a detector. The detector signal generated through this is used for rendering.
Electron-beam devices exhibit a high spatial resolution, which is achieved through a very small diameter of the primary electron beam in the plane of the object. The closer the object is positioned to the objective lens of the electron-beam device, the better the resolution. To detect the secondary or backscattered electrons, the detector is placed here within the objective lens or in a region between the objective lens and the beam generator. Furthermore, the greater the electrons of the primary electron beam are initially accelerated in the electron-beam device and slowed down to a desired final energy at the end in the objective lens or in the region between the objective lens and the object, the better the resolution.
Electron-beam devices that exhibit a ring-shaped detector having a relatively large opening are known. This opening is necessary in order not to influence the primary electron beam in the beam path of the electron-beam device and in order to avoid possible contamination. The reverse paths of the secondary and backscattered electrons in the electron-beam device are influenced by the objective lens due to the different energies of the secondary and backscattered electrons. Here, the crossover of the beam of the secondary electrons lies closer to the object to be examined than the crossover of the beam of the backscattered electrons. The beam of the secondary electrons therefore exhibits more divergence than the beam of the backscattered electrons. However, the secondary and backscattered electrons run on such paths such that the majority of the secondary and backscattered electrons passes through the opening of the detector and is consequently not detected.
In DE 198 28 476 A1, an approach is described in order to avoid the above-mentioned disadvantage. In the electron-beam device known from this publication, two detectors for the secondary and backscattered electrons, each detector exhibiting an opening, are staggered with respect to one another along the optical axis of the electron-beam device. The first detector positioned close to the object detects the electrons that emerge from the object under a relatively large solid angle, while the second detector positioned in the region of the beam generator detects the electrons that emerge from the object under a relatively small solid angle and pass through the opening of the first detector provided for the passage of the primary electron beam. In the electron-beam device known from DE 198 28 476 A1, it is a disadvantage, however, that the second detector always detects a large number of secondary as well as backscattered electrons. The signal obtained using this detector is consequently a mixed signal.
It is known that the backscattered electrons must be detected to increase the resolution of the material contrast. To obtain a particularly good material contrast, it is necessary to capture as many backscattered electrons as possible. This is because the number of detected electrons improves the imaging. The dependency of the imaging on the number of detected electrons applies not just to backscattered electrons but to secondary electrons as well. The higher the number of detected secondary electrons, the better the imaging. A detection of electrons, selected according to secondary and backscattered electrons, is advantageous for carrying out a rendering selected according to the type of electrons.
A scanning electron microscope that separately captures secondary electrons and backscattered electrons is known from US 2002/0185599 A1. For this purpose, two detectors are provided in the scanning electron microscope, which exhibits a beam generator for generating an electron beam and an objective lens for focusing the electron beam on the object. One detector is positioned in the region of the object and detects backscattered electrons. The other detector is positioned above the objective lens and detects secondary electrons. The position of the detector for detecting backscattered electrons must be determined here by simulation, however, which is extremely costly. Moreover, simulations are always fraught with errors so that optimal detection of electrons cannot be achieved.
Accordingly, it is desirable to specify an electron-beam device having a detector system, with which it is possible to make a selection in a simple manner, in particular according to backscattered electrons and secondary electrons. At the same time, as many electrons as possible may be detected using the detector system.